"We will know when quantum computers exist when Satoshis coins move."
https://marketrebellion.com/why-quantum-computing-is-not-a-threat-to-bitcoin/Satoshi knew that one day quantum computers will exist and will be able to move the early mined coins (P2PK) and created an unofficial prize competition to accelerate the development.
Satoshi:
However, if something happened and the signatures were compromised (perhaps integer factorization is solved, quantum computers?), then even agreeing upon the last valid block would be worthless.
True, if it happened suddenly. If it happens gradually, we can still transition to something stronger. When you run the upgraded software for the first time, it would re-sign all your money with the new stronger signature algorithm. (by creating a transaction sending the money to yourself with the stronger sig)
Nobody is asking why he did not move and is not moving these early mined unmoved P2PK coins:
https://bitslog.com/2013/04/17/the-well-deserved-fortune-of-satoshi-nakamoto/https://bitcointalk.org/index.php?topic=175996.0Our guess is that he knew that the early mined coins will be moved one day. So he created a 'prize competition'. Otherwise he could move the coins to quantum resistant P2PKH addresses, but he did not and is not doing.
The only question is:
Who will win the race and get the early coins?
Quantum computing or solving the "Satoshi Prize Competition".
Nobody can stop that race.